Internal combustion engines generate power by causing a mixture of air and combustible fuel, such as gasoline, to ignite and burn in one or more combustion chambers, such as the combustion cylinders in an automobile engine. Conventionally, combustible fuel has been directed into the combustion chambers in vapor form using either a carburetor or a fuel injector. Common fuel injectors can be either continuous or pulsed. The continuous fuel injectors direct the combustible vapor into an intake manifold, and when an intake valve opens, the vapor is drawn into the combustion chamber by a piston. The pulsed fuel injectors direct fuel vapor on command into either a region upstream of each intake valve or directly into the combustion chambers. Both of these fuel delivery systems are highly developed, well known, and have been in use for decades.
On occasion, especially when an internal combustion engine is operated under a high load condition, the fuel vapor can ignite while the fuel vapor is still being compressed within the combustion chamber and prior to the firing of the spark plug. This preignition, which is characterized by a distinct knocking sound, can cause excessive engine wear as well as decrease the performance of the engine.